More than a "replacement" for the Sabbath, therefore, Sunday is its fulfillment, and in a certain sense its extension and full expression in the ordered unfolding of the history of salvation, which reaches its culmination in Christ.
It is the duty of Christians therefore to remember that, although the practices of the Jewish Sabbath are gone, surpassed as they are by the "fulfillment" which Sunday brings, the underlying reasons for keeping "the Lord's Day" holy — inscribed solemnly in the Ten Commandments — remain valid, though they need to be reinterpreted in the light of the theology and spirituality of Sunday:
It is crucially important that all the faithful should be convinced that they cannot live their faith or share fully in the life of the Christian community unless they take part regularly in the Sunday Eucharistic assembly. Therefore, also in the particular circumstances of our own time, Christians will naturally strive to ensure that civil legislation respects their duty to keep Sunday holy.
Likewise, "in celebrating this annual cycle of the mysteries of Christ, the holy Church venerates with special love the Blessed Virgin Mary, Mother of God, united forever with the saving work of her Son"… I entrust this Apostolic Letter to the intercession of the Blessed Virgin, that it may be received and put into practice by the Christian community. Without in any way detracting from the centrality of Christ and his Spirit, Mary is always present in the Church's Sunday..